The present invention relates to a process for cleaning rolling oils contaminated with particles viz., rolling fines originating from metals, especially light weight metals being processed into a semi-finished strip product.
During cold rolling e.g. cold rolling aluminum, rolling fines are unavoidably formed in the mixed friction range. These fine aluminum particles, &gt;0.1 .mu.m in diameter, which are partially retained in the oil film on the rolled (hard) surface of the foil are referred to as smudge.
The larger fraction of the aluminum rolling fines formed during rolling is washed from the surface of the work rolls (roll roughness) into the recycled oil by the rolling oil (coolant and lubricant) sprayed onto the rolls.
These aluminum particles, which are formed as a result of friction, comprise an oil film that is physisorbed on the aluminum oxide surface layer of the metallic particle.
The aluminum particles suspended in the rolling oil exhibit like electrostatic surface surface charges (zeta potential) and so are not able to settle out under normal gravitation.
The rolling fines, present in the rolling oil as a suspension, exibit a distribution that is characteristic of the diameter of the particles. The total amount of rolling fines in the rolling oil is traditionally referred to as oxide ash, and can be determined gravimetrically (DIN-EN 7) or photometrically (% g/g).
A conventional method for cleaning rolling oils is by solid/liquid filtration. Particular preference is given to the Schneider filter. This fully automated multi-chamber vacuum plate type filter, consumes large amounts of filter paper strip and filtering agents (sand, organic absorbants).
Other cleaning systems that in use are e.g. filtering candles (sand-filled wire candles) and matting candle filters.
These filtration units require auxiliary materials such as e.g. filtration sand or organic filtration solids. In each case the disposal of these auxiliary materials is problematic and expensive.
By employing centrifuges it is possible to separate out only the coarser Al fines (larger than 0.5 .mu.m). Combinations of centrifuges and filtration units have low efficiency as the length of time that the auxiliary materials can be employed is determined by the fraction of the finest aluminum fines.
If foil is to be rolled in superimposed pairs using cleaned rolling oil, it is essential to employ chamber type centrifuges after the cleaning operation, especially after a Schneider filter. Traces of "filter sand" that are carried over are precipitated out by the gravitational field of the centrifuge. During cold rolling, traces of "filter sand" in the rolling oil can lead to severe surface defects in the foil (fine porosity, commas, streaks etc.).
Chemical coagulation is known, for example from DE-PS 26 13 878, for cleaning rolling oils. The warm contaminated oil is passed through a coagulator and 0.5 to 1.5 liter of 15 to 25% aqueous sodium carbonate added at a throughput of 400 to 1200 liters per hour after which the coagulum is centrifuged off. Small amounts of hydrogen gas is formed in this process.
The known processes for cleaning rolling oils are not satisfactory in every respect; it is considered a disadvantage that, as only a fraction of the rolling oil is cleaned in parallel with the rest of the circulating rolling oil, complete removal of fines is not possible. Furthermore it is not possible to fully automate the cleaning process. The use of aqueous coagulating agents increases the risk of corrosion. The coagulum i.e. the precipitated colorant is not inhibited and can give off hydrogen.